Protocol for the synthesis of perovskite nanocrystal thin films via in situ crystallization method

The field of halide perovskites currently faces the challenge of finding an efficient approach for producing highly efficient and stable perovskite nanocrystals (PNCs). Here, we present a protocol for the annealing-free and antisolvent-free synthesis of PNCs. We describe the steps for preparing the PNCs precursor solutions. We then detail the procedures to control crucial processing parameters, such as the role of precursor concentration and the creation of humidity-controlled chambers, which allow achieving precise control over the final nanocrystals size. For complete details on the use and execution of this protocol, please refer to Noguera-Gómez et al.1.

Pressure-Induced Phase Transition versus Amorphization in Hybrid Methylammonium Lead Bromide Perovskite

The crystal structure of the CH₃NH₃PbBr₃ perovskite has been investigated under high-pressure conditions by synchrotron-based powder X-ray diffraction. We found that after the previously reported phase transitions in CH₃NH₃PbBr₃ (Pm3̅m→Im3̅→Pmn2₁), which occur below 2 GPa, there is a third transition to a crystalline phase at 4.6 GPa. This transition is reported here for the first time contradicting previous studies, which reported amorphization of CH₃NH₃PbBr₃ between 2.3 and 4.6 GPa. Our X-ray diffraction measurements show that CH₃NH₃PbBr₃ remains crystalline up to at least 7.6 GPa, the highest pressure covered by experiments. The new high-pressure phase is also described by the space group Pmn2₁; however, the transition involves abrupt changes in the unit-cell parameters and a 3% decrease of the unit-cell volume. Our conclusions are confirmed by optical-absorption experiments, by visual observations, and by the fact that pressure-induced changes up to 10 GPa are reversible. The optical studies also allow for the determination of the pressure dependence of the band-gap energy, which is discussed using the structural information obtained from X-ray diffraction.

Solvent and A-Site Cation Control Preferred Crystallographic Orientation in Bromine-Based Perovskite Thin Films

Preferred crystallographic orientation in polycrystalline films is desirable for efficient charge carrier transport in metal halide perovskites and semiconductors. However, the mechanisms that determine the preferred orientation of halide perovskites are still not well understood. In this work, we investigate crystallographic orientation in lead bromide perovskites. We show that the solvent of the precursor solution and organic A-site cation strongly affect the preferred orientation of the deposited perovskite thin films. Specifically, we show that the solvent, dimethylsulfoxide, influences the early stages of crystallization and induces preferred orientation in the deposited films by preventing colloidal particle interactions. Additionally, the methylammonium A-site cation induces a higher degree of preferred orientation than the formamidinium counterpart. We use density functional theory to show that the lower surface energy of the (100) plane facets in methylammonium-based perovskites, compared to the (110) planes, is the reason for the higher degree of preferred orientation. In contrast, the surface energy of the (100) and (110) facets is similar for formamidinium-based perovskites, leading to lower degree of preferred orientation. Furthermore, we show that different A-site cations do not significantly affect ion diffusion in bromine-based perovskite solar cells but impact ion density and accumulation, leading to increased hysteresis. Our work highlights the interplay between the solvent and organic A-site cation which determine crystallographic orientation and plays a critical role in the electronic properties and ionic migration of solar cells.

Printed dry electrode for neuromuscular electrical stimulation (NMES) for e-textile

Muscle atrophy is a well-known consequence of immobilization and critical illness, leading to prolonged rehabilitation and increased mortality. In this study, we develop a solution to preserve muscle mass using customized biocompatible neuromuscular electrical stimulation (NMES) device. Commercially available NMES solutions with gel-based electrodes often lead to skin irritation. We demonstrate the printing of conducting electrodes on a compressive stocking textile that can be used for more than seven days without observing any inflammation. This solution consists of a dry and biocompatible electrode directly integrated into the textile with good mechanical compatibility with skin (Young's modulus of 0.39 MPa). The surface roughness of the underlying substrate plays a significant role in obtaining good print quality. Electrochemical Impedance Spectroscopy (EIS) analysis showed that the printed electrode showed better performance than the commercial ones based on a matched interfacial performance and improved series resistance. Furthermore, we investigated our NMES solution in a hospital setting to evaluate its effectiveness on muscle atrophy, with promising results.

A Geometrically Well-Defined and Systematically Tunable Experimental Model to Transition from Planar to Mesoporous Perovskite Solar Cells

A series of perovskite solar cells with systematically varying surface area of the interface between n-type electron conducting layer (TiO₂) and perovskite are prepared by using an ordered array of straight, cylindrical nanopores generated by anodizing an aluminum layer evaporated onto a transparent conducting electrode. A series of samples with pore length varied from 100 to 500 nm are compared to each other and complemented by a classical planar cell and a mesoporous counterpart. All samples are characterized in terms of morphology, chemistry, optical properties, and performance. All samples absorb light to the same degree, and the increased interface area does not generate enhanced recombination. However, the short circuit current density increases monotonically with the specific surface area, indicating improved charge extraction efficiency. The importance of the slow interfacial rearrangement of ions associated with planar perovskite cells is shown to decrease in a systematic manner as the interfacial surface area increases. The results demonstrate that planar and mesoporous cells obey to the same physical principles and differ from each other quantitatively, not qualitatively. Additionally, the study shows that a significantly lower TiO₂ surface area compared to mesoporous TiO₂ is needed for an equal charge extraction.

Reassigning the Pressure-Induced Phase Transitions of Methylammonium Lead Bromide Perovskite

The high-pressure crystal structure evolution of CH₃NH₃PbBr₃ (MAPbBr₃) perovskite has been investigated by single-crystal X-ray diffraction and synchrotron-based powder X-ray diffraction. Single-crystal X-ray diffraction reveals that the crystal structure of MAPbBr₃ undergoes two phase transitions following the space-group sequence: Pm3̅m → Im3̅ → Pmn2₁, unveiling the occurrence of a nonpolar/polar transition (Im3̅ → Pmn2₁). The transitions take place at around 0.8 and 1.8 GPa, respectively. This result contradicts the previously reported phase transition sequence: Pm3̅m → Im3̅ →Pnma. In this work, the crystal structures of each of the three phases are determined from single-crystal X-ray diffraction analysis, which is later supported by Rietveld refinement of powder X-ray diffraction patterns. The pressure dependence of the crystal lattice parameters and unit-cell volumes are determined from the two aforementioned techniques, as well as the bulk moduli for each phase. The bandgap behavior of MAPbBr₃ has been studied up to around 4 GPa, by means of single-crystal optical absorption experiments. The evolution of the bandgap has been well explained using the pressure dependence of the Pb-Br bond distance and Pb-Br-Pb angles as determined from single-crystal X-ray diffraction experiments.

Spray-driven halide exchange in solid-state CsPbX3 nanocrystal films

CsPbI₃ perovskite nanocrystals (NCs) are promising building blocks for photovoltaics and optoelectronics. However, they exhibit an essential drawback in the form of phase stability: α-phase, with a ∼1.80 eV bandgap, can easily experience a phase transition to a non-radiative orthorhombic δ-phase in an ambient environment. This leads to the need to carry out the CsPbI₃-based device fabrication in an inert atmosphere, which is technologically inconvenient and expensive. One of the most successful approaches proposed to overcome this problem is synthesizing mixed halide CsPbBr_3-xI_x NCs to improve the stability of the α-phase perovskite structure. However, the formation of high-quality thin films of CsPbBr_3-xI_x NCs with high PLQY is challenging owing to the degradation of their optical properties after deposition on a substrate. This work presents spray coating to carry out a solid-state anion exchange in CsPbBr₃ NCs thin films at ambient conditions with low-demanding reaction conditions. This constitutes a novel open-air and annealing-free technology to manufacture CsPbBr_3-xI_x NC thin films with high optical quality and record high photoluminescence quantum yields (PLQY) based on spray-driven halide (Br^- to I^-) anion exchange in a solid-state phase. Besides, tunable emission wavelengths between 520 and 670 nm can be obtained from CsPbBr_3-xI_x NC films using accurate tuning volumes of HI solution sprayed over the initial surface of CsPbBr₃ film to provide the halide exchange. The optical quality of the halide-exchanged PNCs films remains practically identical to that of initial Br-containing layers, with a remarkable PLQY enhancement after anion exchange, from ∼61% for CsPbBr₃ thin films emitting at 520 nm to ∼84% for mixed halide CsPbBr_3-xI_x film emitting at 640 nm. The huge potential of the system is confirmed by demonstrating a low-threshold amplified spontaneous emission.

Understanding equivalent circuits in perovskite solar cells. Insights from drift-diffusion simulation

Perovskite solar cells (PSCs) have reached impressively high efficiencies in a short period of time; however, the optoelectronic properties of halide perovskites are surprisingly complex owing to the coupled ionic-electronic charge carrier dynamics. Electrical impedance spectroscopy (EIS) is a widely used characterization tool to elucidate the mechanisms and kinetics governing the performance of PSCs, as well as of many other semiconductor devices. In general, equivalent circuits are used to evaluate EIS results. Oftentimes these are justified via empirical constructions and the real physical meaning of the elements remains disputed. In this perspective, we use drift-diffusion numerical simulations of typical thin-film, planar PSCs to generate impedance spectra avoiding intrinsic experimental difficulties such as instability and low reproducibility. The ionic and electronic properties of the device, such as ion vacancy density, diffusion coefficients, recombination mechanism, etc., can be changed individually in the simulations, so their effects can be directly observed. We evaluate the resulting EIS spectra by comparing two commonly used equivalent circuits with series and parallel connections respectively, which result in two signals with significantly different time constants. Both circuits can fit the EIS spectra and by extracting the values of the elements of one of the circuits, the values of the elements of the other circuit can be unequivocally obtained. Consequently, both can be used to analyse the EIS of a PSC. However, the physical meaning of each element in each circuit could differ. EIS can produce a broad range of physical information. We analyse the physical interpretation of the elements of each circuit and how to correlate the elements of one circuit with the elements of the other in order to have a direct picture of the physical processes occurring in the device.

Amplified spontaneous emission in thin films of quasi-2D BA3MA3Pb5Br16 lead halide perovskites

Quasi-2D (two-dimensional) hybrid perovskites are emerging as a new class of materials with high photoluminescence yield and improved stability compared to their three-dimensional (3D) counterparts. Nevertheless, despite their outstanding emission properties, few studies have been reported on amplified spontaneous emission (ASE) and a thorough understanding of the photophysics of these layered materials is still lacking. In this work, we investigate the ASE properties of multilayered quasi-2D BA₃MA₃Pb₅Br₁₆ films through the dependence of the photoluminescence on temperature and provide a novel insight into the emission processes of quasi-2D lead bromide perovskites. We demonstrate that the PL and ASE properties are strongly affected by the presence, above 190 K, of a minor fraction of the high temperature (HT) phase. This phase dominates the PL spectra at low excitation density and strongly affects the ASE properties. In particular, ASE is only present between 13 K and 230 K, and, at higher temperatures, it is suppressed by absorption of charge transfer states of the HT phase. Our results improve the understanding of the difficulties to obtain ASE at room temperature from these quasi-2D materials and are expected to guide possible materials improvement in order to exploit their excellent emission properties also for the realization of low threshold optically pumped lasers.

ZnS Ultrathin Interfacial Layers for Optimizing Carrier Management in Sb2S3-based Photovoltaics

Antimony chalcogenides represent a family of materials of low toxicity and relative abundance, with a high potential for future sustainable solar energy conversion technology. However, solar cells based on antimony chalcogenides present open-circuit voltage losses that limit their efficiencies. These losses are attributed to several recombination mechanisms, with interfacial recombination being considered as one of the dominant processes. In this work, we exploit atomic layer deposition (ALD) to grow a series of ultrathin ZnS interfacial layers at the TiO2/Sb2S3 interface to mitigate interfacial recombination and to increase the carrier lifetime. ALD allows for very accurate control over the ZnS interlayer thickness on the ångström scale (0-1.5 nm) and to deposit highly pure Sb2S3. Our systematic study of the photovoltaic and optoelectronic properties of these devices by impedance spectroscopy and transient absorption concludes that the optimum ZnS interlayer thickness of 1.0 nm achieves the best balance between the beneficial effect of an increased recombination resistance at the interface and the deleterious barrier behavior of the wide-bandgap semiconductor ZnS. This optimization allows us to reach an overall power conversion efficiency of 5.09% in planar configuration.

Efficient Vacuum Deposited P-I-N Perovskite Solar Cells by Front Contact Optimization

Hole transport layers (HTLs) are of fundamental importance in perovskite solar cells (PSCs), as they must ensure an efficient and selective hole extraction, and ohmic charge transfer to the corresponding electrodes. In p-i-n solar cells, the ITO/HTL is usually not ohmic, and an additional interlayer such as MoO₃ is usually placed in between the two materials by vacuum sublimation. In this work, we evaluated the properties of the MoO₃/TaTm (TaTm is the HTL N4,N4,N4″,N4″-tetra([1,1′-biphenyl]-4-yl)-[1,1′:4′,1″-terphenyl]-4,4″-diamine) hole extraction interface by selectively annealing either MoO₃ (prior to the deposition of TaTm) or the bilayer MoO₃/TaTm (without pre-treatment on the MoO₃), at temperature ranging from 60 to 200°C. We then used these p-contacts for the fabrication of a large batch of fully vacuum deposited PSCs, using methylammonium lead iodide as the active layer. We show that annealing the MoO₃/TaTm bilayers at high temperature is crucial to obtain high rectification with low non-radiative recombination, due to an increase of the electrode work function and the formation of an ohmic interface with TaTm.

Short Photoluminescence Lifetimes in Vacuum-Deposited CH3NH3PbI3 Perovskite Thin Films as a Result of Fast Diffusion of Photogenerated Charge Carriers

It is widely accepted that a long photoluminescence (PL) lifetime in metal halide perovskite films is a crucial and favorable factor, as it ensures a large charge diffusion length leading to a high power conversion efficiency (PCE) in solar cells. It has been recently found that vacuum-evaporated CH₃NH₃PbI₃ (eMAPI) films show very short PL lifetimes of several nanoseconds. The corresponding solar cells, however, have high photovoltage (>1.1 V) and PCEs (up to 20%). We rationalize this apparent contradiction and show that eMAPI films are characterized by a very high diffusion coefficient D, estimated from modeling the PL kinetics to exceed 1 cm²/s. Such high D values are favorable for long diffusion length as well as fast transport of carriers to film surfaces, where they recombine nonradiatively with surface recombination velocity S ∼ 10⁴ cm/s. Possible physical origins leading to the high D values are also discussed.

Room-Temperature Cubic Phase Crystallization and High Stability of Vacuum-Deposited Methylammonium Lead Triiodide Thin Films for High-Efficiency Solar Cells

Methylammonium lead triiodide (MAPI) has emerged as a high-performance photovoltaic material. Common understanding is that at room temperature, it adopts a tetragonal phase and it only converts to the perfect cubic phase around 50-60 °C. Most MAPI films are prepared using a solution-based coating process, yet they can also be obtained by vapor-phase deposition methods. Vapor-phase-processed MAPI films have significantly different characteristics than their solvent-processed analogous, such as relatively small crystal-grain sizes and short excited-state lifetimes. However, solar cells based on vapor-phase-processed MAPI films exhibit high power-conversion efficiencies. Surprisingly, after detailed characterization it is found that the vapor-phase-processed MAPI films adopt a cubic crystal structure at room temperature that is stable for weeks, even in ambient atmosphere. Furthermore, it is demonstrated that by tuning the deposition rates of both precursors during codeposition it is possible to vary the perovskite phase from cubic to tetragonal at room temperature. These findings challenge the common belief that MAPI is only stable in the tetragonal phase at room temperature.

Perovskite-Perovskite Homojunctions via Compositional Doping

One of the most important properties of semiconductors is the possibility of controlling their electronic behavior via intentional doping. Despite the unprecedented progress in the understanding of hybrid metal halide perovskites, extrinsic doping of perovskite remains nearly unexplored and perovskite-perovskite homojunctions have not been reported. Here we present a perovskite-perovskite homojunction obtained by vacuum deposition of stoichiometrically tuned methylammonium lead iodide (MAPI) films. Doping is realized by adjusting the relative deposition rates of MAI and PbI₂, obtaining p-type (MAI excess) and n-type (MAI defect) MAPI. The successful stoichiometry change in the thin films is confirmed by infrared spectroscopy, which allows us to determine the MA content in the films. We analyzed the resulting thin-film junction by cross-sectional scanning Kelvin probe microscopy (SKPM) and found a contact potential difference (CPD) of 250 mV between the two differently doped perovskite layers. Planar diodes built with the perovskite-perovskite homojunction show the feasibility of our approach for implementation in devices.

Effects of Frequency Dependence of the External Quantum Efficiency of Perovskite Solar Cells

Perovskite solar cells are known to show very long response time scales, on the order of milliseconds to seconds. This generates considerable doubt over the validity of the measured external quantum efficiency (EQE) and consequently the estimation of the short-circuit current density. We observe a variation as high as 10% in the values of the EQE of perovskite solar cells for different optical chopper frequencies between 10 and 500 Hz, indicating a need to establish well-defined protocols of EQE measurement. We also corroborate these values and obtain new insights regarding the working mechanisms of perovskite solar cells from intensity-modulated photocurrent spectroscopy measurements, identifying the evolution of the EQE over a range of frequencies, displaying a singular reduction at very low frequencies. This reduction in EQE is ascribed to additional resistive contributions hindering charge extraction in the perovskite solar cell at short-circuit conditions, which are delayed because of the concomitant large low-frequency capacitance.

Interfacial Modification for High-Efficiency Vapor-Phase-Deposited Perovskite Solar Cells Based on a Metal Oxide Buffer Layer

Vacuum deposition is one of the most technologically relevant techniques for the fabrication of perovskite solar cells. The most efficient vacuum-based devices rely on doped organic contacts, compromising the long-term stability of the system. Here, we introduce an inorganic electron-transporting material to obtain power conversion efficiencies matching the best performing vacuum-deposited devices, with open-circuit potential close to the thermodynamic limit. We analyze the leakage current reduction and the interfacial recombination improvement upon use of a thin (<10 nm) interlayer of C₆₀, as well as a more favorable band alignment after a bias/ultraviolet light activation process. This work presents an alternative for organic contacts in highly efficient vacuum-deposited perovskite solar cells.

High Stability Bilayered Perovskites through Crystallization Driven Self-Assembly

In this manuscript we reveal the formation of bilayered hybrid perovskites of a new lower dimensional perovskite family, (CHMA)₂(MA)_n-1Pb_nI₃ with n = 1-5, with high ambient stability via its crystallization driven self-assembly process. The spun-coated perovskite solution tends to crystallize and undergo phase separation during annealing, resulting in the formation of 2D/3D bilayered hybrid perovskites. Remarkably, this 2D/3D hybrid perovskites possess striking moisture resistance and displays high ambient stability up to 65 days. The bilayered approach in combining 3D and 2D perovskites could lead to a new era of perovskite research for high-efficiency photovoltaics with outstanding stability, with the 3D perovskite providing excellent electronic properties while the 2D perovskite endows it moisture stability.

Atomically Altered Hematite for Highly Efficient Perovskite Tandem Water-Splitting Devices

Photoelectrochemical (PEC) cells are attractive for storing solar energy in chemical bonds through cleaving of water into oxygen and hydrogen. Although hematite (α-Fe₂ O₃ ) is a promising photoanode material owing to its chemical stability, suitable band gap, low cost, and environmental friendliness, its performance is limited by short carrier lifetimes, poor conductivity, and sluggish kinetics leading to low (solar-to-hydrogen) STH efficiency. Herein, we combine solution-based hydrothermal growth and a post-growth surface exposure through atomic layer deposition (ALD) to show a dramatic enhancement of the efficiency for water photolysis. These modified photoanodes show a high photocurrent of 3.12 mA cm^-2 at 1.23 V versus RHE, (>5 times higher than Fe₂ O₃ ) and a plateau photocurrent of 4.5 mA cm^-2 at 1.5 V versus RHE. We demonstrate that these photoanodes in tandem with a CH₃ NH₃ PbI₃ perovskite solar cell achieves overall unassisted water splitting with an STH conversion efficiency of 3.4 %, constituting a new benchmark for hematite-based tandem systems.

Surface Recombination and Collection Efficiency in Perovskite Solar Cells from Impedance Analysis

The large diffusion lengths recurrently measured in perovskite single crystals and films signal small bulk nonradiative recombination flux and locate the dominant carrier recombination processes at the outer interfaces. Surface recombination largely determines the photovoltaic performance, governing reductions under short-circuit current and open-circuit voltage. Quantification of recombination losses is necessary to reach full understanding of the solar cell operating principles. Complete impedance model is given, which connects capacitive and resistive processes to the electronic kinetics at the interfaces. Carrier collection losses affecting the photocurrent have been determined to equal 1%. Photovoltage loss is linked to the decrease in surface hole density, producing 0.3 V reduction with respect to the ideal radiative limit. Our approach enables a comparison among different structures, morphologies, and processing strategies of passivation and buffer layers.

Perovskite Materials for Light-Emitting Diodes and Lasers

Organic-inorganic hybrid perovskites have cemented their position as an exceptional class of optoelectronic materials thanks to record photovoltaic efficiencies of 22.1%, as well as promising demonstrations of light-emitting diodes, lasers, and light-emitting transistors. Perovskite materials with photoluminescence quantum yields close to 100% and perovskite light-emitting diodes with external quantum efficiencies of 8% and current efficiencies of 43 cd A(-1) have been achieved. Although perovskite light-emitting devices are yet to become industrially relevant, in merely two years these devices have achieved the brightness and efficiencies that organic light-emitting diodes accomplished in two decades. Further advances will rely decisively on the multitude of compositional, structural variants that enable the formation of lower-dimensionality layered and three-dimensional perovskites, nanostructures, charge-transport materials, and device processing with architectural innovations. Here, the rapid advancements in perovskite light-emitting devices and lasers are reviewed. The key challenges in materials development, device fabrication, operational stability are addressed, and an outlook is presented that will address market viability of perovskite light-emitting devices.

Nanostructuring Mixed-Dimensional Perovskites: A Route Toward Tunable, Efficient Photovoltaics

2D perovskites is one of the proposed strategies to enhance the moisture resistance, since the larger organic cations can act as a natural barrier. Nevertheless, 2D perovskites hinder the charge transport in certain directions, reducing the solar cell power conversion efficiency. A nanostructured mixed-dimensionality approach is presented to overcome the charge transport limitation, obtaining power conversion efficiencies over 9%.

Carbon nanotubes as an efficient hole collector for high voltage methylammonium lead bromide perovskite solar cells

A high open circuit voltage (V(OC)) close to 1.4 V under AM 1.5, 100 mW cm(-2) conditions is achieved when carbon nanotubes (CNTs) are used as a hole conductor in methyl ammonium lead bromide (MAPbBr3) perovskite solar cells. Time-resolved photoluminescence and impedance spectroscopy investigations suggest that the observed high V(OC) is a result of the better charge extraction and lower recombination of the CNT hole conductor. Tandem solar cells with all perovskite absorbers are demonstrated with a MAPbBr3/CNT top cell and a MAPbI3 bottom cell, achieving a V(OC) of 2.24 V in series connection. The semitransparent and high voltage MAPbBr3/CNT solar cells show great potential for applications in solar cell windows, tandem solar cells and solar driven water splitting.

Lead-Free MA2CuCl(x)Br(4-x) Hybrid Perovskites

Despite their extremely good performance in solar cells with efficiencies approaching 20% and the emerging application for light-emitting devices, organic-inorganic lead halide perovskites suffer from high content of toxic, polluting, and bioaccumulative Pb, which may eventually hamper their commercialization. Here, we present the synthesis of two-dimensional (2D) Cu-based hybrid perovskites and study their optoelectronic properties to investigate their potential application in solar cells and light-emitting devices, providing a new environmental-friendly alternative to Pb. The series (CH3NH3)2CuCl(x)Br(4-x) was studied in detail, with the role of Cl found to be essential for stabilization. By exploiting the additional Cu d-d transitions and appropriately tuning the Br/Cl ratio, which affects ligand-to-metal charge transfer transitions, the optical absorption in this series of compounds can be extended to the near-infrared for optimal spectral overlap with the solar irradiance. In situ formation of Cu(+) ions was found to be responsible for the green photoluminescence of this material set. Processing conditions for integrating Cu-based perovskites into photovoltaic device architectures, as well as the factors currently limiting photovoltaic performance, are discussed: among them, we identified the combination of low absorption coefficient and heavy mass of the holes as main limitations for the solar cell efficiency. To the best of our knowledge, this is the first demonstration of the potential of 2D copper perovskite as light harvesters and lays the foundation for further development of perovskite based on transition metals as alternative lead-free materials. Appropriate molecular design will be necessary to improve the material's properties and solar cell performance filling the gap with the state-of-the-art Pb-based perovskite devices.

Efficient photoluminescent thin films consisting of anchored hybrid perovskite nanoparticles

Methylammonium lead bromide nanoparticles are synthetized with a new bifunctional ligand which allows anchoring of the nanoparticles on a variety of conducting and semiconducting surfaces, showing bright photoluminescence with a quantum yield exceeding 50%.

Open circuit potential build-up in perovskite solar cells from dark conditions to 1 sun

The high open-circuit potential (Voc) achieved by perovskite solar cells (PSCs) is one of the keys to their success. The Voc analysis is essential to understand their working mechanisms. A large number of CH3NH3PbI3-xClx PSCs were fabricated on single large-area substrates and their Voc dependencies on illumination intensity, I0, were measured showing three distinctive regions. Similar results obtained in Al2O3 based PSCs relate the effect to the compact TiO2 rather than the mesoporous oxide. We propose that two working mechanisms control the Voc in PSCs. The rise of Voc at low I0 is determined by the employed semiconductor n-type contact (TiO2 or MgO coated TiO2). In contrast, at I0 close to AM1.5G, the employed oxide does not affect the achieved voltage. Thus, a change of regime from an oxide-dominated EFn (as in the dye sensitized solar cells) to an EFn, directly determined by the CH3NH3PbI3-xClx absorber is suggested.

Inorganic Halide Perovskites for Efficient Light-Emitting Diodes

Lead-halide perovskites have transcended photovoltaics. Perovskite light-emitting diodes (PeLEDs) emerge as a new field to leverage on these fascinating semiconductors. Here, we report the first use of completely inorganic CsPbBr3 thin films for enhanced light emission through controlled modulation of the trap density by varying the CsBr-PbBr2 precursor concentration. Although pure CsPbBr3 films can be deposited from equimolar CsBr-PbBr2 and CsBr-rich solutions, strikingly narrow emission line (17 nm), accompanied by elongated radiative lifetimes (3.9 ns) and increased photoluminescence quantum yield (16%), was achieved with the latter. This is translated into the enhanced performance of the resulting PeLED devices, with lower turn-on voltage (3 V), narrow electroluminescence spectra (18 nm) and higher electroluminescence intensity (407 Cd/m(2)) achieved from the CsBr-rich solutions.

Revealing the Role of TiO2 Surface Treatment of Hematite Nanorods Photoanodes for Solar Water Splitting

Ultrathin TiO2 is deposited on conventional hydrothermal grown hematite nanorod arrays by atomic layer deposition (ALD). Significant photoelectrochemical water oxidation performance improvement is observed when the ALD TiO2-treated samples are annealed at 650 °C or higher temperatures. The electrochemical impedance spectroscopy (EIS) study shows a surface trap-mediated charge transfer process exists at the hematite-electrolyte interface. Thus, one possible reason for the improvement could be the increased surface states at the hematite surface, which leads to better charge separation, less electron-hole recombination, and hence, greater improvement of photocurrent. Our Raman study shows the increase in surface defects on the ALD TiO2-coated hematite sample after being annealed at 650 °C or higher temperatures. A photocurrent of 1.9 mA cm(-2) at 1.23 V (vs RHE) with a maximum of 2.5 mA cm(-2) at 1.8 V (vs RHE) in 1 M NaOH under AM 1.5 simulated solar illumination is achieved in optimized deposition and annealing conditions.

Core-shell hematite nanorods: a simple method to improve the charge transfer in the photoanode for photoelectrochemical water splitting

We report a simple method to produce a stable and repeatable photoanode for water splitting with a core-shell hematite (α-Fe2O3) nanorods system by combining spray pyrolysis and hydrothermal synthesis. Impedance spectroscopy revealed passivation of the surface states by the shell layer, which results in an increase of the charge injection through the hematite conduction band. In pristine hematite more holes are accumulated on the surface and the charge transfer to the electrolyte occurs through surface states, whereas in the core-shell hematite photoanode the majority of hole transfer process occurs through the valence band. As a result the photoactivity of the core-shell nanorods, 1.2 mA cm(-2), at 1.23 V vs RHE, is twice that of pristine hematite nanorods. The alteration of the interface energetics is supported by TEM, showing that the crystallinity of the surface has been improved by the deposition of the shell.

Perovskite Solar Cells: Beyond Methylammonium Lead Iodide

Organic-inorganic lead halide based perovskites solar cells are by far the highest efficiency solution-processed solar cells, threatening to challenge thin film and polycrystalline silicon ones. Despite the intense research in this area, concerns surrounding the long-term stability as well as the toxicity of lead in the archetypal perovskite, CH3NH3PbI3, have the potential to derail commercialization. Although the search for Pb-free perovskites have naturally shifted to other transition metal cations and formulations that replace the organic moiety, efficiencies with these substitutions are still substantially lower than those of the Pb-perovskite. The perovskite family offers rich multitudes of crystal structures and substituents with the potential to uncover new and exciting photophysical phenomena that hold the promise of higher solar cell efficiencies. In addressing materials beyond CH3NH3PbI3, this Perspective will discuss a broad palette of elemental substitutions, solid solutions, and multidimensional families that will provide the next fillip toward market viability of the perovskite solar cells.

Loading of mesoporous titania films by CH3NH3PbI3 perovskite, single step vs. sequential deposition

The sequential deposition method enables a better pore filling and a more conformal and structured capping layer of perovskite on top of the TiO₂. The contact areas between the HTM/perovskite and HTM/TiO₂ are, thus, minimized and the shunting is reduced, which is reflected on the FF and V_oc.

Modulating light propagation in ZnO–Cu2O-inverse opal solar cells for enhanced photocurrents

The advantages of employing an interconnected periodic ZnO morphology, i.e. an inverse opal structure, in electrodeposited ZnO/Cu₂O devices are presented.

Hole-transporting small molecules based on thiophene cores for high efficiency perovskite solar cells

Two new electron-rich molecules, 2,3,4,5-tetra[4,4'-bis(methoxyphenyl)aminophen-4"-yl]-thiophene (H111) and 4,4',5,5'-tetra[4,4'-bis(methoxyphenyl)aminophen-4"-yl]-2,2'-bithiophene (H112), which contain thiophene cores with arylamine side groups, are reported. When used as the hole-transporting material (HTM) in perovskite-based solar cell devices, power conversion efficiencies of up to 15.4% under AM 1.5G solar simulation were obtained. This is the highest efficiency achieved with HTMs not composed of 2,2',7,7'-tetrakis(N,N'-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) and its isomers. Both HTMs, especially H111, have great potential to replace expensive spiro-OMeTAD given their much simpler and less expensive syntheses.

Incorporation of Cl into sequentially deposited lead halide perovskite films for highly efficient mesoporous solar cells

Organic-inorganic lead halide perovskites have been widely used as absorbers on mesoporous TiO2 films as well as thin films in planar heterojunction solar cells, yielding very high photovoltaic conversion efficiencies. Both the addition of chloride and sequential deposition methods were successfully employed to enhance the photovoltaic performance. Here, both approaches are combined in a sequential method by spincoating PbCl2 + PbI2 on a mesoporous TiO2 film followed by the perovskite transformation. The role of Cl in determining the optical, electrical, structural and morphological properties is correlated with the photovoltaic performance. The highest photovoltaic efficiency of 14.15% with the V(oc), FF and J(sc) being 1.09 V, 0.65 and 19.91 mA cm(-2) respectively was achieved with 10 mol% of PbCl2 addition due to an increase of the film conductivity induced by a better perovskite morphology. This is linked to an improvement of the hysteresis and reproducibility of the solar cells.

Lead-free halide perovskite solar cells with high photocurrents realized through vacancy modulation

Lead free perovskite solar cells based on a CsSnI3 light absorber with a spectral response from 950 nm is demonstrated. The high photocurrents noted in the system are a consequence of SnF2 addition which reduces defect concentrations and hence the background charge carrier density.

Iron pyrite thin film counter electrodes for dye-sensitized solar cells: high efficiency for iodine and cobalt redox electrolyte cells

Iron pyrite has been the material of interest in the solar community due to its optical properties and abundance. However, the progress is marred due to the lack of control on the surface and intrinsic chemistry of pyrite. In this report, we show iron pyrite as an efficient counter electrode (CE) material alternative to the conventional Pt and poly(3,4-ethylenedioxythiophene (PEDOT) CEs in dye-sensitized solar cells (DSSCs). Pyrite film CEs prepared by spray pyrolysis are utilized in I3(-)/I(-) and Co(III)/Co(II) electrolyte-mediated DSSCs. From cyclic voltammetry and impedance spectroscopy studies, the catalytic activity is found to be comparable with that of Pt and PEDOT in I3(-)/I(-) and Co(III)/Co(II) electrolyte, respectively. With the I3(-)/I(-) electrolyte, photoconversion efficiency is found to be 8.0% for the pyrite CE and 7.5% for Pt, whereas with Co(III)/Co(II) redox DSSCs, efficiency is found to be the same for both pyrite and PEDOT (6.3%). The excellent performance of the pyrite CE in both the systems makes it a distinctive choice among the various CE materials studied.

Facile water-based spray pyrolysis of earth-abundant Cu2FeSnS4 thin films as an efficient counter electrode in dye-sensitized solar cells

A novel approach to produce earth-abundant Cu2FeSnS4 (CFTS) thin film using spray pyrolysis of nontoxic aqueous precursors followed by sulfurization is reported. The CFTS phase formation was confirmed by both Raman spectroscopy and X-ray diffraction techniques. Hall measurements of these films reveal p-type conductivity with good charge carrier density and mobilities appropriate for solar harvesting devices. To the best of our knowledge, this is the first report on the electrical properties of solution-processed Cu2FeSnS4 thin films estimated using Hall measurements. Dye-sensitized solar cells (DSSC) fabricated with CFTS thin film as a photocathode in iodine/iodide electrolyte exhibit good power conversion efficiency, 8.03%, indicating that CFTS would be a promising cheaper alternative to replace Pt as a counter electrode in DSSCs.

Laminated carbon nanotube networks for metal electrode-free efficient perovskite solar cells

Organic-inorganic metal halide perovskite solar cells were fabricated by laminating films of a carbon nanotube (CNT) network onto a CH3NH3PbI3 substrate as a hole collector, bypassing the energy-consuming vacuum process of metal deposition. In the absence of an organic hole-transporting material and metal contact, CH3NH3PbI3 and CNTs formed a solar cell with an efficiency of up to 6.87%. The CH3NH3PbI3/CNTs solar cells were semitransparent and showed photovoltaic output with dual side illuminations due to the transparency of the CNT electrode. Adding spiro-OMeTAD to the CNT network forms a composite electrode that improved the efficiency to 9.90% due to the enhanced hole extraction and reduced recombination in solar cells. The interfacial charge transfer and transport in solar cells were investigated through photoluminescence and impedance measurements. The flexible and transparent CNT network film shows great potential for realizing flexible and semitransparent perovskite solar cells.

Engineering a Cu2O/NiO/Cu2MoS4 hybrid photocathode for H2 generation in water

We report a scalable process for fabricating a multiple-layer hybrid photocathode, namely Cu2O/NiO/Cu2MoS4, for H2 generation in water. In pH 5 solution and under 1 sun illumination, the photocathode showed interesting photocatalytic properties. The onset photocurrent was recorded at +0.45 V vs. RHE, while at 0 V vs. RHE, a photocurrent density of 1.25 mA cm(-2) was obtained. It was found that the NiO interlayer enhances charge transfer from the Cu2O light harvester to the Cu2MoS4 hydrogen evolution reaction electrocatalyst which in turn accelerates charge transfer at the electrode/electrolyte interface, and therefore improves the photocatalytic properties of the Cu2O photocathode.

High efficiency electrospun TiO₂ nanofiber based hybrid organic-inorganic perovskite solar cell

The good electrical and morphological characteristics of TiO₂ nanofibers and the high extinction coefficient of CH₃NH₃PbI₃ perovskite are combined to obtain a solar cell with a power conversion efficiency of 9.8%. The increase of the film thickness dramatically diminishes the performance due to the reduction in porosity of the TiO₂ nanofiber framework. The optimum device (∼413 nm film thickness) is compared to a planar device, where the latter produces higher V(oc) but lower J(sc), and consequently lower efficiency at all measured light intensities.

High efficiency solid-state sensitized solar cell-based on submicrometer rutile TiO2 nanorod and CH3NH3PbI3 perovskite sensitizer

We report a highly efficient solar cell based on a submicrometer (~0.6 μm) rutile TiO2 nanorod sensitized with CH3NH3PbI3 perovskite nanodots. Rutile nanorods were grown hydrothermally and their lengths were varied through the control of the reaction time. Infiltration of spiro-MeOTAD hole transport material into the perovskite-sensitized nanorod films demonstrated photocurrent density of 15.6 mA/cm(2), voltage of 955 mV, and fill factor of 0.63, leading to a power conversion efficiency (PCE) of 9.4% under the simulated AM 1.5G one sun illumination. Photovoltaic performance was significantly dependent on the length of the nanorods, where both photocurrent and voltage decreased with increasing nanorod lengths. A continuous drop of voltage with increasing nanorod length correlated with charge generation efficiency rather than recombination kinetics with impedance spectroscopic characterization displaying similar recombination regardless of the nanorod length.

High performance PbS Quantum Dot Sensitized Solar Cells exceeding 4% efficiency: the role of metal precursors in the electron injection and charge separation

Here we report the preparation of high performance Quantum Dot Sensitized Solar Cells (QDSCs) based on PbS-CdS co-sensitized nanoporous TiO2 electrodes. QDs were directly grown on the TiO2 mesostructure by the Successive Ionic Layer Absorption and Reaction (SILAR) technique. This method is characterized by a fast deposition rate which involves random crystal growth and poor control of the defect states and lattice mismatch in the QDs limiting the quality of the electrodes for photovoltaic applications. In this work we demonstrate that the nature of the metallic precursor selected for SILAR has an active role in both the QD's deposition rate and the defect's distribution in the material, with important consequences for the final photovoltaic performance of the device. For this purpose, acetate and nitrate salts were selected as metallic precursors for the SILAR deposition and films with similar absorption properties and consequently with similar density of photogenerated carriers were studied. Under these conditions, ultrafast carrier dynamics and surface photovoltage spectroscopy reveal that the use of acetate precursors leads to higher injection efficiency and lower internal recombination due to contribution from defect states. This was corroborated in a complete cell configuration with films sensitized with acetate precursors, achieving unprecedented photocurrents of ~22 mA cm(-2) and high power conversion efficiency exceeding 4%, under full 1 sun illumination.

Effect of Organic and Inorganic Passivation in Quantum-Dot-Sensitized Solar Cells

The effect of semiconductor passivation on quantum-dot-sensitized solar cells (QDSCs) has been systematically characterized for CdS and CdS/ZnS. We have found that passivation strongly depends on the passivation agent, obtaining an enhancement of the solar cell efficiency for compounds containing amine and thiol groups and, in contrast, a decrease in performance for passivating agents with acid groups. Passivation can induce a change in the position of TiO2 conduction band and also in the recombination rate and nature, reflected in a change in the β parameter. Especially interesting is the finding that β, and consequently the fill factor can be increased with the passivation treatment. Applying this strategy, record cells of 4.65% efficiency for PbS-based QDSCs have been produced.

Recombination in Organic Bulk Heterojunction Solar Cells: Small Dependence of Interfacial Charge Transfer Kinetics on Fullerene Affinity

We investigate the causes for obtaining higher open-circuit voltage in solar cells that use a fullerene with a smaller electron affinity. Using impedance spectroscopy technique, we show that the change of fullerene LUMO energy has very little influence on the kinetic rate of charge transfer across the interface. In terms of the Marcus theory, large reorganization energy values govern the recombination kinetic rate, which is only slightly dependent on the fullerene LUMO energy, and also depends weakly on the energy location of recombining carriers within the broad density of states. Since the recombination rate is very similar in the different devices, we conclude that the larger open-circuit voltage is due to the larger donor HOMO/acceptor LUMO offset.